Per p. 191 of the Player's Handbook, you can't move through a hostile creature's space unless they're two sizes larger or smaller than you. You can move through a nonhostile creature's space but can't willingly stop there. Either way, another creature's space is considered difficult terrain.
The guardian of faith spell creates a "Large spectral guardian" in "an unoccupied space of your choice".
Should the guardian be regarded as a creature for purposes of moving through its space? Or is it an impassable barrier?
Nowhere in the spell description is it described as a creature, nor is it given an AC and hit points that would allow it to be attacked as one. It doesn't get opportunity attacks against creatures that move away from it. On the other hand, it's explicitly described as Large, and I can think of no other reason why that information would be relevant.
Is there an official answer to this, or is it up to the GM?
Best Answer
It isn't clear, so it's up to the DM
Nothing in the spell text for guardian of faith (PHB, 246) states that it is a creature. It states (my emphasis):
However, it also doesn't clarify what it actually is. Compare this to Bigby's hand (PHB, 218), which uses similar language (emphasis mine):
But note that is also explicitly states:
Guardian of faith never explicitly says it's either a creature or an object. Without that information, it's really up to a DM to determine what it is.
Most creatures and objects have HP and AC
One thing to consider when making your ruling is that creatures and objects have AC and HP. The guardian does not.
That's a strong case for it not being a creature or an object, and not falling under the creature space rules or the physical object issues.
It just being visible as a shield and sword seems like it's possible to step through it.
Why say what size it is?
The need for knowing how much space it controls is necessary in order to determine how close someone is to it to trigger the dexterity save.